1. Technical Field
The invention relates generally to simulations of electrical and electronic circuits and more particularly to a method for using such simulations for performing failure mode and effects analysis of motor vehicle electrical systems.
2. Description of the Problem
Failure mode and effects analysis (FMEA), combined with computer based simulation of electrical circuits, has provided engineers a valuable virtual tool for fault effect and propagation analysis and design verification. An electrical circuit may be modeled using conventional symbols to represent passive and active components. Lines represent the wires that connect components. Components and wires have parameters for which values are specified. Once completed the model is used by the program to emulate a system design. The system's normal limits of operation are readily estimated and by introducing faults to the circuit and determining whether the fault generates a failure, engineers are aided in providing robust, fault tolerant systems for the motor vehicle industry. The emulation tool can provide a detailed map in spreadsheet format for evaluation of simulation runs.
The generation of useful data is eased for engineers by providing them the ability to model efficiently and accurately an electrical system. Engineers would be further assisted by gaining the ability to incorporate component tolerances into the analysis to obtain the expected normal operating limits and by gaining the ability to alter the circuit topology to emulate the introduction of faults. Providing an intuitive and easy to use interface for accomplishing these latter two tasks would be valuable.
System performance is determined by obtaining the system's operating variables in terms of signal voltage and current levels, over time, at strategic nodes in the circuit. However, in terms of specification of device tolerances for a system intended for mass production, normal variation of component operating parameter values is as of much or more interest than is operation of an ideal circuit. Real devices vary in quality and seldom operate in the same environment. Operating data is expected to vary over a range of values and accordingly evaluation of a circuit typically involves repeated simulation runs.
Typically the limits of range in output values is estimated through setting component parameters to the upper and lower limits of their tolerances. This requires only two runs. However, there is no guarantee that this technique pushes the system to the extremes of operational response since concurrent changes in component parameters may cancel one another. The parameters for individual components may be varied randomly. While this process is readily automated, the approach requires a large number of simulation runs to produce a statistically high level of confidence that the “limits of normal operation” have been found. Another approach is to allow the user to specify component parameters individually and assign those values over runs trusting the user to develop an intuition or heuristic method for selecting changes likely to produce a failure in a given system.
The term “failure” has been a source of confusion. In some cases the term may refer to externally set design limits, and thus be arbitrary. Here it must be determined if the specified tolerances for components are acceptable or whether the system can fail without a component fault.
The invention described here is best employed with the SaberHamess™ mixed signal simulator, which may be used to analyze electrical, mechanical and hydraulic systems. It is labeled a mixed signal simulator due to its integration of two different analysis methods. The first method utilizes algorithms to solve equations associated with analog circuitry. The second method employs event driven states which are useful for modeling digital circuitry.
SaberHarness™ mixed signal simulator is constructed in three layers, the simulator core and first and second levels of user interface. The simulator core operates on command line options and a netlist. The simulator netlist conforms to common computer modeling languages, which provide for interconnectivity and physical modeling. The first level interface (or sketch interface) also provides a conveniently used tool to interact with the simulator and to build a database larger in scope than the netlist. The second level interface (harness), provides a high degree of functionality with the inclusion of specialized models for wires, connectors and splices. Both the harness and sketch interfaces were written to be enhanced by users.